This invention relates to active matrix electroluminescent display devices comprising an array of electroluminescent display pixels.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of an electroluminescent material, for example a semiconducting conjugated polymer, sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising a display element and a driving device for controlling the current through the display elements. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and EP-A-0717446. Unlike active matrix liquid crystal display devices in which display elements are capacitive and therefore take virtually no current and allow a drive (data) signal voltage to be stored on the capacitance for the whole frame time electroluminescent display elements need to continuously pass current to generate light. A driving device of a pixel, usually comprising a TFT (thin film transistor), is responsible for controlling the current through the display element. The brightness of the display element is dependent to the current flowing through it. During an address period for a pixel, a drive (data) signal determining the required output from the display element is applied to the pixel and stored as a corresponding voltage on a storage capacitor which is connected to, and controls the operation of, the current controlling drive device with the voltage stored on the capacitor serving to maintain operation of the driving device in supplying current through the display element during the subsequent drive period, corresponding to a frame period, until the pixel is addressed again.
Typically, the pixels are connected to sets of row and column address conductors through which selection, (scanning), signals and analogue voltage data signals respectively are supplied by a peripheral drive circuit, each row of pixels being selected in turn in a respective row address period by means of a selection signal applied to its associated row conductor and with the data signals for the pixels of the selected row being applied via the column conductors. The data signals can be provided by a column driver circuit comprising silicon integrated circuits (IC) chips. Each chip has a limited number of individual, spaced, output contacts. Each column conductor is connected to a respective chip output and consequently a large number of chips would normally be required. For example, if there are 800 pixels in a row and a chip is capable of providing 100 outputs then 8 chips are needed to supply the 800 column conductors involved.
The electroluminescent display elements of all the pixels in a respective row, or column, are connected, through their associated drive devices to a common current line. The storage capacitors of the pixels are also connected to these common lines and such sharing of the current lines leads to a further problem in that voltage drops can occur along these lines in operation which has the effect of producing a kind of cross-talk.
It is an object of the present invention to provide an improved active matrix electroluminescent display device.
According to the present invention there is provided an active matrix electroluminescent display device comprising a row and column array of pixels carried on a substrate, each pixel comprising an electroluminescent display element and a driving device for controlling current through the display element in a drive period based on a data signal applied in a preceding row address period, the display element being connected via the driving device to a current line common to a row of pixels, and a peripheral drive circuit connected to the pixel array which drive circuit generates and applies data signals to each row of pixels in respective row address periods via a set of address conductors connected to the array of pixels and comprises at least one drive IC having a plurality of outputs, which is characterised in that the at least one drive IC is connected to the set of address conductors through a multiplexing circuit which is integrated on the substrate and is operable to apply data signals from each output of the drive IC to a respective plurality of address conductors in the set in sequence in the row address period, and in that the drive circuit is arranged to prevent current flowing through the display elements of a row of pixels during its respective row address period.
Through the integration of a multiplexing circuit on the device substrate operable in this manner, considerable cost savings are possible as fewer drive ICs are required for a device having a given number of columns of pixels. With a multiplexing ratio of 4:1 for example, wherein each group of address conductors supplied by a respective drive IC output comprises four address conductors, the cost of the drive ICs needed is reduced by 75% in comparison to the case in which a single IC output is connected exclusively to a respective single address conductor. Using the same thin film fabrication technology employed for fabricating the pixels the integrated multiplexing circuit is provided at little or no additional expense and can be formed at the same time as the thin film elements of the pixels using common thin film layers and comprising similar thin film circuit elements such as TFTs and conductor lines. Multiplexing switches used in the multiplexing circuit are preferably of the same type as used in the pixel array, for example p or n type polysilicon TFTs. Consequently it is possible to produce the thin film circuitry on the device substrate forming the pixel array and the multiplexing circuit using standard thin film technology involving the deposition and patterning of various conductive, dielectric and semiconductive layers. With the pixel circuits and the integrated multiplexer circuit using the same type of switching device, e.g. either p or n channel polysilicon TFTs, fabrication of the array together with the multiplexing circuit is considerable simplified, typically requiring only 5 or 6 mask processes rather than 9 or 10 mask processes as normally required to produce both p and channel (CMOS) TFTs.
However, both p and n channel type TFTs could be used which would enable circuitry such as shift registers requiring CMOS circuits to be integrated as well on the device substrate. Shift registers could be used in the column drive circuit and/or in the row drive (selection) circuit. The benefits of using both p and n (CMOS) devices in allowing extended integration would need to be considered in relation to the more complicated (higher mask count) fabrication processes necessary.
The pixels of each row are addressed with their data signals in a respective row address period during which the multiplexing circuit operates to supply data signals in time division manner to the pixels of each group in the row in sequence. By preventing current flowing through the display elements during the period of their addressing then problems with cross-talk effect caused by voltage drops occurring in their shared current line due to inherent resistance are avoided. Such prevention can be accomplished by ensuring that the display elements are zero or reversed biased during the full, entire, row address period rather than merely the portion thereof in which individual pixels are addressed. To this end, the potential applied to the common current line may be switched or a switching device, e.g. another TFT, may be connected in series with the display element of each pixel that is operable to disconnect the display element from the current line for the duration of the row address period.